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<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <script>
        // var flag = true;
        var flag1 = false;
        // console.log(flag + 1);
        console.log(flag1 + 1);
        // 如果声明变量未赋值，则输出是underfind类型
        var str;
        console.log(str);
        // 一下效果等同于上面
        var variable = undefined;
        console.log(variable)

        // var variable = undefined;
        console.log(variable + 'pink')
        console.log(variable + 1)
            // underfind和数字相加输出结果是NaN
            // null值
        var space = null;
        console.log(space + 'pink');
        console.log(space + 1);
        // 输出为1
    </script>
</head>

<body>

</body>

</html>